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Published July 2012 | public
Journal Article

Transient creep effects and the lubricating power of water in materials ranging from paper to concrete and Kevlar


A diverse class of viscous materials, which includes familiar materials such concrete, wood, and Kevlar, exhibit surprising, counterintutive properties under internal moisture content fluctuations. In test after test over the past 50 years, the viscosity of these materials is observed to decrease, often dramatically, during wetting and drying. The key characteristics of the observed viscous softening are: the decrease in viscosity is temporary, and depending on the specimen size can be greatly delayed with respect to the associated change in weight; the decrease in viscosity is absent under steady state flow. Based on recent research on the properties of water and other polar fluids confined by hydrophilic surfaces, we provide a physical explanation and propose a constitutive law. The resulting model accurately captures the interplay between the pore fluid movement and macroscopic constitutive properties in totality. The model is verified against published data for the creep of paper sheets exposed to cyclic moisture conditions. Experimental data of different materials under similar boundary conditions are compared using a new metric, the creep rate factor. The results further reinforce the idea that nanoscale movement of water enhances the internal 'lubrication' of the studied materials, interpreted as loosening of the hydrogen bonds.

Additional Information

© 2012 Elsevier Ltd. Received 31 May 2011. Revised 24 February 2012. Accepted 3 March 2012. Available online 16 March 2012. This work was partly supported by the Initiative for Sustainable Energy at Northwestern University, the US DOE Grant no. DE-FG02-08ER15980, the US AFOSR Grant no. FA9550-08-1-1092, and the US NSF Grant no. CMMI-1060087. This support is gratefully acknowledged. I.V. would also like to thank Dr. Jia-Liang Le for his insight on the topic of size effect during the preparation of the paper. We also thank the anonymous reviewers for their insight on polymeric materials and generally constructive comments aimed at improving the paper.

Additional details

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